Multiple distinct iridium anomalies have been observed in dinosaur eggshells in the K/T boundary sections of the Nanxiong Basin, Guangdong Province, South China. The eggshells collected at and near the fossil-pollen-defined K/T boundary interval show iridium increases of about 19 and 28 times, respectively, above the background level. The enrichment of other trace elements in the eggshells occurs mostly at and near the interval. The distribution of Ir-bearing levels proves the existence of at least six Ir-delivering events from the latest Cretaceous into the earliest Paleocene. The enrichment of Ir and other trace elements in eggshells may have been caused by the assimilation of these elements into the dinosaur body through food, and then into the eggs laid by them. Two types of pathological development, i.e. variation in eggshell thickness and eggshell microstructure, have been observed from the basin. It seems that they occur frequently during the K/T transition. The physiological mechanisms producing pathologic dinosaur eggshells are evaluated in the light of homologous phenomena occurring in living birds. On this basis, it is concluded that the formation of pathologic dinosaur eggshells was caused by anomalous trace element concentrations. The extinction of the dinosaurs in the Nanxiong Basin did not occur instantaneously, but spread out within 250 ka with major extinction beginning at the boundary interval. The cause may have been environmental poisoning and adverse changes in climatic conditions as indicated by the geochemical analyses of the dinosaur eggshells. A repeating short- and long-term geochemically induced environmental stress adversely affected the reproductive process and contributed to the extinction of the dinosaurs.
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